1. Technical Field
The present invention relates generally to a technique for energy conversion using a hierarchical structure, in which a primary multi-channel includes one or more unit channels radially arranged and the unit channel includes an inflow channel, an outflow channel, and a secondary multi-channel having one or more channels arranged in parallel, according to a technique for converting mechanical energy into electric energy to generate streaming potential and streaming current when liquid is allowed to flow through the microfluidic channel with charged wall by applying pressure drop, based on an electrokinetic principle that causes an electric field and a flow field to be combined. The present invention improves output power and flow stability, so that it may be applied to nano locomotion, MEMS devices operation, ubiquitous power supply, and self-power harvesting by human body.
2. Description of the Related Art
An electrokinetic phenomenon is caused due to an interaction of a charged solid surface with a liquid containing electrolyte ions, in which there are electro-osmosis and electrophoresis in which liquids or particles dispersed in the liquid move when an electric field is applied thereto, and, vice versa, a streaming potential in which an electric field is generated when liquids move. Compared to the electro-osmosis and electrophoresis that have greatly contributed to the progress of analytical science and technology over the last 100 years, the streaming potential has started to receive attention in a possible application as an emergent energy conversion and harvesting technology, thus microscale liquid flow has recently become a field of active study.
For example, Olthuis et al. [W. Olthuis, B. Schippers, J. Eijkel, A. van den Berg, “Energy from streaming current and potential”, Sensors and Actuators B, 111-112, 385-389, 2005] reported a streaming potential, a streaming current, and energy conversion results obtained using porous glass filter with a pore size in a range of 1.0 to 1.6 μm for different external resistances. Mansouri et al. [A. Mansouri, S. Bhattacharjee and L. Kostiuk, “High-power electrokinetic energy conversion in a glass microchannel array”, Lab Chip, 12, 4033-4036, 2012] proposed a channel network in which both ends of a porous glass filter were coated with a gold foil having a thickness of a nanometer level to allow it to function as an electrode. They proved the practical feasibility of an apparatus for energy conversion based on electrokinetic effects by generating output power above 1 mW.
Further, Chun et al. [M.-S. Chun, M. S. Shim, D. K. Choi, “Electrokinetic Micro Power Cell Using Pile-Up Disk Type Microfluidic-Chip with Multi-Channel”, U.S. Pat. No. 7,709,126, May 4, 2010] invented a micro power cell by stacking one or more disk type microfluidic-chips made of polydimethylsiloxane (PDMS) material, in which multi-channels were formed by a micro-electromechanical system (MEMS) process to improve output power and to realize uniform flow distribution. Subsequently, Myung-Suk Chun [“Silicon microfluidic-chip with parallel multi-channel and micro/nano energy system using the chip”, Korean Patent No. 10-1050141, Jul. 12, 2011] invented an energy system capable of achieving enhanced energy according to an external resistance, by composing multiple unit cells in which disk type microfluidic-chips made of silicon material having higher charged property than PDMS are stacked.
The electrokinetic principle in which a streaming potential and a streaming current are generated is as follows. When a channel wall is charged, counter-ions having an opposite sign to the charge of the channel wall are gathered near the wall due to movements of electrolyte ions dissolved in a liquid to form an electric double layer and electrostatic potential distribution. Herein, a thickness of the electric double layer is a measure of electrostatic interaction and is inversely proportional to a square root of a concentration of all ions dissolved in the liquid. That is, in a case of distilled-deionized water having an extremely low ion concentration of 10−7 mol, the electric double layer has the maximum thickness of about 1 μm and therefore the wall surface is strongly charged. On the other hand, as the ion concentration increases, the electric double layer becomes thin and therefore the wall surface becomes weakly charged. Another measure of the electrostatic interaction is surface potential which is defined as an electrostatic potential at the wall. The larger the surface potential value, the stronger the charged property of the wall surface. The surface potential value can be regarded as a zeta potential value experimentally measured, which changes according to pH (or, the negative of the logarithm of a hydrogen ion concentration).
When the incompressible liquid of viscosity μ flows into the channel with a charged wall surface at a velocity of u by a pressure gradient (∇p) between both ends of the channel, a flow field in a steady state is represented by Equation 1 below.∇p=μ∇2u+ρe∇ϕ  [Equation 1]Here, a velocity of liquid in a square channel having a width W, a height H, and a length L exists only in an axial direction uz. ρe∇ϕ is the external force due to movements of ions and defined as a product of a net charge density ρe of co-ions and counter-ions per unit volume in the electric double layer formed near the channel wall and a gradient ∇ϕ (=Δϕ/L) of the streaming potential Δϕ formed by movements of the counter ions.
More specifically, in the electric double layer composed of a fixed layer and a diffuse layer, when the counter-ions in the diffuse layer having an opposite sign to the charge of the channel wall move toward a downstream end of the channel, a streaming current Is is generated along both ends of the channel in a direction of the liquid flow.
The potential gradient due to ionic charges accumulated on a downstream end of the channel builds an electric field and thus generates a streaming potential, and simultaneously the electric field generates a conduction current Ic along the fixed layer of the electric double layer in a direction opposite to the liquid flow.
Any ion transports in the channel can be divided into a transport by a pressure gradient and a transport by a potential gradient, which can be quantified by the Nernst-Planck equation. Since the net current consisting of streaming current and conduction current is conserved inside the channel at steady state, it is represented by a relation of Is+Ic=0, whereby the streaming potential Δϕ can be finally obtained by the following Equation 2.
                    Δϕ        =                              LI            s                                              2              ⁢                              (                                  W                  +                  H                                )                            ⁢                              λ                s                                      +                          ∫                                                ∑                  i                                ⁢                                                                                                    Λ                        i                        2                                            ⁢                                              e                        2                                            ⁢                                              N                        A                                                              kT                                    ⁢                                      D                    i                                    ⁢                                      n                    i                                    ⁢                  dA                                                                                        [                  Equation          ⁢                                          ⁢          2                ]            Here, λs is a surface conductivity in the channel wall, NA is Avogadro's number, Di is a diffusion coefficient of ion i, k is Boltzmann constant, Λi is a valence of ion i, e is a unit charge, and ni is the number concentration (1/m3) of ion i dissolved in the liquid.